I’ve been designing a lot of monsters in the past year, and
I’ve noticed that I tend to do so from one of two directions regarding the art:
either my text comes before ordering the art, or my text comes after the art
exists. Both directions warrant some careful consideration to make sure the
creature’s powers align with its image. A GM often shows a picture of the monster
to the players (“you see…THIS awful thing!”), but the players also get a sense
of the monster from the way they see it behave and attack. As there’s a strong
disconnect when these two don’t line up (“wait, the statue is 24 feet tall? It
looks like a little garden gnome!”), so making sure these match is important no
matter which direction you’re designing from.

Usually, text comes before the art. That is, the monster
will be fully designed—including both its stat block and its flavor text—prior to
any art being ordered for it. In this case, the monster description must have visual hooks that can be communicated
to the artist. If the gains some sort of camouflage ability in the forest, the
art order should state that the monster is colored in greens and browns,
perhaps with leaf patterns. If it has a powerful stinger attack that injects
poison, the art order should include some references to the type of stinger—like
a bulbous scorpion stinger or a straighter wasp stinger—the creature should
have. There are some nuances to the visual experience you should keep in mind.
For example, a scorpion stinger generally communicates “this has venom that can
kill you,” while a wasp stinger generally communicates “this will cause pain.”
The art order is also a good time to communicate the monster’s size; a good
artist can make a large creature seem really big in the image (perhaps with a
wagon or well-known animal for size reference), or can make a tiny creature
seem really small. But the artist doesn’t know unless you communicate what the
rules say.

Sometimes, I’ve been asked to invent stats for a creature
that already has art. In this case, the
stats should primarily convey attacks that the art clearly shows. If the
image is of an earwig-shaped monster with a screaming face, then its stats
should have a pincer attack and some sort of sound-based attack, and it
probably climbs or burrows. It’s good monster design to avoid loading up a
monster with a dozen special attacks—monsters aren’t on the table for long and
don’t often have a chance to use more than a couple tricky powers. When
designing a monster to match an existing image, I prefer to give it three special powers: two that the art
suggests, and one that matches the theme of the art but doesn’t seem obvious
from the image. For the earwig monster, I did a little Wikipedia research
to learn that some earwigs shoot a stinky yellow goo at predators. That’s a suitably
gross surprise for the PCs that meet this monster, and something the art need not show.

(Note that I’m assuming here that I’m responsible for creating the art. There
are many people in this industry who are more talented than I am and can do all
of design, development, and art on their own. My advice above is only
tangentially applicable to these paragons when they handle it all themselves!)